The present invention relates to a battery capacity measuring device that measures battery capacity of a rechargeable battery and a battery capacity measuring method for use in the battery capacity measuring device.
Nickel-metal hydride rechargeable batteries and lithium ion rechargeable batteries are used for vehicle-mounted power supplies for use in electric vehicles, hybrid vehicles, or the like, because these rechargeable batteries have high energy density. Such a rechargeable battery is typically configured as an assembled battery including a plurality of battery modules each of which includes a plurality of single batteries.
In rechargeable batteries, the present ratio of the battery capacity to the initial battery capacity is referred to as “capacity retention rate”. The capacity retention rate of the rechargeable battery effects the traveling distance of an electric vehicle or the like. It is thus desirable that the capacity retention rate of the rechargeable battery be accurately calculated. International Patent Publication No. WO 2014/054796 describes an example of a battery capacity measuring device that calculates the capacity retention rate with higher accuracy.
The battery capacity measuring device described in International Patent Publication No. WO 2014/054796 includes a rechargeable battery of which degree of deterioration is measured, a memory unit that stores specific information including a measured frequency, a measured temperature, and initial limit capacity of a rechargeable battery having the same specification as the rechargeable battery to be measured, and a temperature measurement unit that measures the temperature of the rechargeable battery. Additionally, the battery capacity measuring device includes a power supply unit that applies an AC signal having a measured frequency of 0.5 mHz or greater and less than 10 mHz to the rechargeable battery having a measured temperature of 40° C. or greater and 70° C. or less based on the information in the memory unit, and a measurement unit that measures the impedance of the rechargeable battery using the AC signal applied by the power supply unit (see region dc of impedance curve L51 in Nyquist plot shown in FIG. 5). The battery capacity measuring device further includes a calculation unit that calculates the degree of deterioration using the limit capacity of the rechargeable battery that is calculated using a measured value obtained by the measurement unit and the initial limit capacity stored in the memory unit.